Variable band width receiver



April 1942- M. s. CROSBY 2,280,570

VARIABLE BAND WIDTH RECEIVER Filed Jan. 8, 1941' 2 Sheets-Sheet l A'ILI'ORNEY April 21', 1942. M. e. CROSBY ,2 0, 7

' 'VARIABLE'BAND WIDTH RECEIVER Fi led Jan. 8, 1941!. 2 Sheets-Sh'eet 2 ATTORNEY Patented Apr. 21, 1942 UNITED ST VARIABLE BAND WIDTH RECEIVER Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 8, 1941, Serial No. 373,539

6 Claims. (Cl. 250-20) This application concerns a variable intermediate-frequency band width frequency modulation receiver wherein the deviation ratio may be changed and the system thereby rendered more adaptable to the reception of weak signals by switching to a lower deviation ratio at the receiver and reducing the deviation at the transmitter. Hence, when the signal is weak, the lower deviation ratio may be used and when the signal is strong enough to allow a higher deviation ratio,

it may be used. The improvement in signals bothered by ignition is especially good in this respect. Any of the known variable intermedi ate-frequency band width methods may be used, for instance, two separate quency filters with a switching system or variable coupling intermediate-frequency transformers.

In one arrangement of automatic deviation ratio adjustment for a frequency modulation receiver, I use a receiver having two intermediatefrequency channels, a broad and narrow one, so that two deviation ratios may be produced. The receiver is set up in such a manner that the narrow intermediate-frequency amplifier is used when the deviation is low in the soft passages of music for instance and the broad intermediate frequency is used in the forte passages.

In Figs. 1, 2, and 3, I have shown several modifications of wavelength modulated wave receiving means including circuits of difierent band 3i pass width with automatic means for deriving the demodulation output from that circuit having a band pass characteristic closest to the width of the band received.

In Fig. 1, a relay is controlled by rectified wave energy to supply output from that receiver whose band pass width is of the order of the Width of the band received.

In Fig.2,rectifiers are arranged and operated to control modulation potential amplifiers in the output of the demodulators of different band widths in such a manner that the output is derived from a circuit having a band pass characteristic of the order of the band pass of the received wave.

Referring to Fig, 1 of the drawings, 4 is a radiant energy pick-up means of any approved type connected with a wave amplifying and, if desired, wave frequency reducing means of the heterodyne type. amplifier in unit 5 is fed to the input of two wavelength modulated wave amplifiers and demodulators it and 12, respectively. One of these amplifiers and demodulators has a band pass such as to accept wave energy with the deviation intermediate-fre- The output of the receiving ratio D=N where N is any number while the other has a band pass characteristic such as to accept wave energy of a deviation ratio D=1.

The output of demodulator ill is supplied by transformer to a diode detector 45. The winding of a relay 54 is connected in series with the diode detector 40. The armature 56 of the relay 54 connects either the output of ID or the output of l2, depending upon its position, to the output means or jack 51. Spring 58 holds the armature against contact in the absence of sufficient energizing current from H).

In operation, when the frequency deviation is low, that is indicating a low amount of modue lation on the wave, the output jack 5! is connected to the output of receiver I2 because the pull of the spring 58 overcomes the pull eiiected by the winding of the relay 54 and the armature is moved to contact 4| by spring 58. When the frequency deviation increases the output of It increases so as to cause increased current to flow in tube in and energize the winding of relay 54 sufficiently to move the armature 56 to contact 5! to connect the output means 51 to the demodulator of higher band pass characteristic in unit Ill.

In this circuit a rectifier is set up to control the choice of two receiver outputs, (the one receiver with a deviation ratio D=1 and the other with a higher deviation ratio D=N). The rectifier it is arranged to operate a relay 54 when the level rises above a certain amount. Let us assume a sample case: Audio band=10 kc., N :10; then when the frequency deviation is less than 10 kc. the springovercomes the pull of the relay magnet so that the output jack is connected to unit 12. For higher frequency deviation, the pull of the relay magnet overcomes the pull of spring 58 so that armature 55 is connected to 5| and the jack 5'! is connected to receiver Ill. Thus, when the signal gets up to a certain level, the pull of the relay magnet overcomes the pull of the spring and pulls the relay tongue 56 to a position to connect the output jack to receiver it.

In the circuit of Fig. 2, the stronger signal biases down the weaker low-deviation-ratio, signal after a certain signal level has been reached, and at the same time or point, the low-deviation receiver rectified output biases up the audio-frequency amplifier of the high-deviation signal. By proper adjustment of rectified bias, the device may be made to switch the output from the low-deviation receiver to the high-deviation-ratio' receiver at the point where the frequency deviation starts to overload the low-deviation-ratio receiver.

In the modification shown in Fig. 2, two frequency modulated wave amplifiers and demodulaters 60 and 10 have their outputs connected by audio frequency amplifiers BI and l l, respectively, to a combining circuit 13 connected with an output means 15. The outputs of the means 69 and H! of different deviation ratio are respectively connected by transformers 18 and 82 to diode rectifiers 84 and 86, respectively. Here, the diode rectifiers are biased by adjustable sources 81 and 89. A point between the cathode 83 and resistance 90 of diode 86 is connected to the modulation potential amplifier 6| to control the gain thereof. A point between the anode 85 of the diode 84 and the diode resistor 9| is connected in a similar manner to the modulation-potentialfrequency amplifier H to control the gain thereof. Both of these gain control circuits may be connected in a well known manner to control electrodes in tubes in said amplifiers to control the gain thereof. Bias battery 92 serves to bias amplifier 6! to cut off, so as to be inoperative in the absence of modulation.

In operation, for low depths of modulation, bias battery 92 makes amplifier 6| inoperative so that output is furnished to 13 from H. Thus, for the lower depths of modulation, the low-deviationratio receiver furnishes the output. When the depth of modulation rises to such a level as would start to overload the low-deviation-ratio receiver, biases 81 and 89 are adjusted to be overcome by the rectified output from transformers 82 and 18, respectively. Hence, at this level, a positive rectified voltage will appear across resistor 90 and a negative voltage across resistor 9|. The positive voltage from 90 is fed through bias battery 92 to amplifier so that the negative bias from 92 is overcome by the positive bias from 90 and amplifier GI becomes operative. At the same time the negative voltage from 9| biases amplifier H toward cutoff and tends to make it inoperative. Thus, at this level, the amplifier from the wide receiver is turned off at the same time that the amplifier for the narrow receiver is turned on. This simultaneous biasing of both amplifiers tends to make the changeover action more rapid than it would be if only one amplifier were biased. Somewhere during the change-over cycle there will be a condition of equal output from both receivers so that combining amplifier 13 is necessary to combine the two outputs for this interval.

In the modification of Fig. 3, I have shown an embodiment in which the voltage to be rectified for control is taken via leads [9 from the combined output. The operation is otherwise essentially the same as that of Fig. 2 except that the diode biasing batteries 81 and 89 are eliminated. Elimination of these batteries causes the changeover to be more gradual and continuous instead of abrupt as in the case of Figs. 1 and 2. Such a gradual and continuous change-over is an alternative adjustment which is advantageous in conditions where the difference in signal-to-noise ratio on the two receivers is rather large and the abrupt change would be annoying.

What is claimed is:

1. In a system for receiving frequency modulated carrier waves, at least two parallel modulated carrier wave transmission channels, each channel being adapted to derive the modulation signal from the modulated carrier wave, one of said channels having a relatively wide pass band characteristic thereby to accommodate modulated carrier waves having a relatively wide frequency deviation range, the second channel having a relatively narrow pass band characteristic, a modulation signal utilization network having connections to each of said channels, a rectifier network connected to rectify the modulation signal output of at least one of said channels, and means responsive to the rectified modulation signal for controlling the efiectiveness of the connections between said utilization network and each of said channels.

2. In a system for receiving frequency modulated carrier waves, at least two parallel modulated carrier wave transmission channels, each channel being adapted to derive the modulation signal from the modulated carrier wave, one of said channels having a relatively wide pass band characteristic thereby to accommodate modulated carrier waves having a relatively wide frequency deviation range, the second channel having a relatively narrow pass band characteristic, a modulation signal utilization network having connections to each of said channels, a rectifier network connected to rectify the modulation signal output of at least one of said channels, and means responsive to the rectified modulation signal for controlling the effectiveness of the connections between said utilization network and each of said channels, said connections comprising parallel modulation signal amplifiers.

In a system for receiving frequency modulated carrier waves, at least two parallel modulat-ed carrier wave transmission channels, each channel being adapted to derive the modulation signal from the modulated carrier wave, one of said channels having a relatively wide pass band characteristic thereby to accommodate modulated carrier waves having a relatively wide frequency deviation range, the second channel having a relatively narrow pass band characteristic, a modulation signal utilization network having connections to each of said channels, a rectifier network connected to rectify the modulation signal output of at least one of said channels, and means responsive to the rectified modulation signal for controlling the eifectiveness of the connections between said utilization network and each of said channels, and said latter control being such that solely the connection between the wide band channel and the utilization network is effective in response to increased modulation signal amplitude.

4. In a system for receiving frequency modulated carrier waves. at least two parallel modulatcd carrier wave transmission channels, each channel being adapted to derive the modulation signal from the modulated carrier wave, one of said channels having a relatively wide pass band characteristic thereby to accommodate modulated carrier waves having a relatively wide frequency deviation range, the second channel having a relatively narrow pass band characteristic, a modulation signal utilization network having connections to each of said channels, a rectifier network connected to rectify the modulation signal output of at least one of said channels, and means responsive to the rectified modulation signal for controlling the efiectiveness of the connections between said utilization network and each of said channels, said rectifier network comprising a diode rectifier coupled to the wide pass band channel output.

5. In combination with a pair of parallel frequency modulated carrier wave transmission channels, each channel including a, demodulator for providing modulation signal output, one of the channels having a pass band to transmit Wide frequency deviations of the carrier, the second having a pass band to transmit narrow frequency deviations, independent modulation signal output connections from the channels, a common utilizing network coupled to both output connections, a separate rectifier coupled to a respective one of the output connections to provide rectified modulation signal voltage, and a control connection from each rectifier to a predetermined one of the said output connections to control the efiectiveness thereof with said rectified voltage.

6. In combination with a pair of parallel frequency modulated carrier wave transmission channels, each channel including a demodulator for providing modulation signal output, one of the channels having a pass band to transmit Wide frequency deviations of the carrier, the second having a pass band to transmit narrow frequency deviations, independent modulation signal output connections from the channels, a common utilizing network coupled to both output connections, a separate rectifier coupled to a respective one of the output connections to provide rec tified modulation signal voltage, and a control connection from each rectifier to a predetermined one of the said output connections to control the effectiveness thereof with said rectified voltage, each of said output connections comprising a modulation signal amplifier, means for biasing each of said rectifiers.

MURRAY G. CROSBY. 

